Light modulator element

ABSTRACT

A device for modulating and a nondestructive readout storage device employing modulation of light transmitted through an irregular ferroelectric crystal before and after the rotation of the vibration plane thereof caused by an applied electric field equal to or larger than the coercive field thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light modulator utilizing thevariation in the orientation of the vibration plane of an irregularferroelectric crystal accompanying the polarization reversal thereof.

2. Description of the Prior Art

There are various conventional optical switching elements such asammonium dihydrogen phosphate (hereinafter referred to as ADP) employingan electrooptical effect and Kerr cells employing birefringence causedwhen a substance such as nitrobenzene is placed in an electric field.All of these elements are such that the intensity of light transmittedthrough these elements is controlled by placing the elements between twopolarizers the vibration planes of which are orthogonal and applyingthereto an electric field. In such elements

1. The quantity of light transmitted therethrough is proportional to theapplied field. A high voltage is necessary for intensifying thebrightness of the transmitted light.

2. Since the quantity of transmitted light is proportional to theapplied field, light is not transmitted when the applied voltage isreduced to zero, that is, these optical elements have no memoryfunction. Therefore, in order to maintain the brightness at a constantvalue, it is necessary to keep the elements impressed with a voltagecorresponding thereto.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical switchingelement having a memory function and capable of controlling switchingtime.

It is another object of the present invention to provide a ferroelectricstorage device having no dependency on voltage, frequency and time.

It is a further object of the present invention to provide a storagedevice wherein information stored in a ferroelectric storage element isnondestructively read out.

It is still another object of the present invention to provide a largecapacity storage device wherein information stored in a ferroelectricstorage element is read out with a high S/N ratio.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1a is a hysteresis loop of polarization versus electric field of aferroelectric material;

FIG. 1b is a hysteresis loop of generated electric charge versus stressof an irregular ferroelectric material;

FIG. 1c is a hysteresis loop of mechanical strain versus electric fieldof an irregular ferroelectric material;

FIG. 1d is a quantity of transmitted light versus voltage characteristicof an irregular ferroelectric material;

FIG. 2 is a diagram showing the change in the dimension of an irregularferroelectric crystal wherein (a) is the state of the crystal with nostress nor applied electric field, and (b) is the state of the crystalwith an applied electric field higher than the coercive field.

FIG. 3 is a part of the indicatrix ellipsoid of a biaxial birefringentcrystal;

FIG. 4 is a diagram schematically showing how white light is polarized;

FIG. 5 is a diagram showing the state of interference of the lightpassed through the device of FIG. 4;

FIG. 6 is a crystal element used for an optical shutter device;

FIG. 7 is an embodiment of the optical shutter device according to theinvention;

FIG. 8 is another embodiment of the invention;

FIG. 9 is an arrangement of electrodes on a storage element according tothe invention;

FIG. 10 is still another embodiment of the invention;

FIG. 11a is a wave form of a readout signal;

FIG. 11b is a current versus time characteristic of a readout currentwhen a storage element is in a "0" state; and

FIG. 11c is a current versus time characteristic of a readout currentwhen a storage element is in a "1" state. .Iadd.

FIG. 12 illustrates an embodiment of the invention employing mechanicalstress applying means. .Iaddend.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Ferroelectric material has generally a hysteresis characteristic asshown in FIG. 1a between an applied field E and an electric polarizationP. In other words, as the electric field applied to the ferroelectricmaterial grows high, the polarization of the ferroelectric materialreaches the state indicated by CA in FIG. 1a. Then, as the applied fieldis gradually reduced, the polarization also becomes reduced, and whenthe applied field exceeds the negative coercive field -E_(c) afterhaving passed through zero, the polarization is reversed. As theintensity of the applied field is further increased in the negativedirection, the polarization reaches the state indicated by DB in FIG.1a.

According to studies on ferroelectrics made by the inventors it wasfound that some kinds of ferroelectrics such as potassium dihydrogenphosphate (hereinafter referred to as KDP) and gadolinium molybdenumoxide (hereinafter referred to as GMO) have the property that when astress of more than a certain value (hereinafter referred to as coercivestress), to say nothing of an electric field of more than a certainvalue (hereinafter referred to as coercive field), is applied to theferroelectrics, the direction of the spontaneous polarization 5 thereofis reversed and, at the same time, the crystal lattice thereof undergoesdeformation as shown in FIGS. 2a and 2b as contrasted to the knownferroelectrics such as triglycine sulfate, lead zircon-titanate andbarium titanate whose spontaneous polarizations are reversed in theirdirection by the application of the coercive field and which undergo nocrystal lattice deformation.

Generally, a crystal having an electric polarization called spontaneouspolarization in the absence of stress and electric field and capable ofbeing reversed in its spontaneous polarization depending on an appliedelectric field as shown in FIG. 1a is conventionally called aferroelectric crystal. In some of the ferroelectric crystals, the strainin the crystal lattice is different depending on the direction of thespontaneous polarization as shown in FIG. 2. Such a ferroelectriccrystal will hereinafter be referred to as an irregular ferroelectriccrystal. The above-mentioned KDP and GMO are examples of irregularferroelectric crystals. Irregular ferroelectrics belong toferroelastoelectrics. In contrast, a ferroelectric crystal of which thestrain in the lattice is independent of the direction of the spontaneouspolarization is called a regular ferroelectric crystal. Referring toFIG. 2 in which h, k and l indicate the length of the edges of thecrystal along the crystallographic axes a, b and c respectively, thecrystal in the state (a) is expanded in a direction perpendicular to thesheet of the drawing, while in the state (b), it is elongated in thehorizontal direction. That is, the crystal in the state (a) correspondsto that in the state (b) rotated 90° around the axis c. With this changein deformation, the tensorial properties of the crystal changeaccordingly.

There are basically two methods of transforming an irregularferroelectric crystal from one state to the other. According to onemethod, a crystal which is in the state of FIG. 2a is give a compressiveforce in the direction of k to cause a strain. If the compressive forceexceeds a certain value, the crystal will be transformed into the stateof FIG. 2b, and the polarity of the electrification on both end surfacesperpendicular to the direction of the spontaneous polarization will bereversed. This phenomenon corresponds to the generation of electriccharge or electromotive force due to a mechanical stress. In this case,the relation between the stress X and the charge density Q is expressedby a hysteresis loop as shown in FIG. 1b, and both polarized statesopposite to each other are stable in the absence of an electric field ora mechanical stress. The other method of transforming the state of thecrystal is to impose an electric field on the crystal in the directionopposite to that of the spontaneous polarization to reverse thepolarization as described previously. Accompanying the reversal of thepolarization there occurs a change in strain as shown in FIG. 2. In thiscase, the relation between the electric field and the mechanical strainis as shown in FIG. 1c.

Needless to say, in an irregular ferroelectric crystal, the relationbetween the mechanical stress and strain also shows a hysteresis loopsimilar to those of FIGS. 1b and 1c. Such a mechanical behavior isentirely different from elasticity or plasticity of ordinary materials,and it is a property rather comparable with ferroelectricity orferromagnetism. Therefore, it may be called "ferroelasticity," and anirregular ferroelectric crystal may be said to be ferroelectric as wellas ferroelastic material. According to the investigation made by theinventors, it has been found that some crystals among the crystalsbelonging to the point groups mm2, 2-I and 2-II fall within the categoryof the irregular ferroelectric material. The following table Ienumerates these crystals under the respective group indices imm2, i2-Iand i2-II.

                  TABLE I                                                         ______________________________________                                        Point                                                                         group     Material                                                            ______________________________________                                        imm2      KDP, GMO.                                                           i2-I      Not yet discovered.                                                 i2-II     Rochelle salt, cadmium ammonium sulfate,                                      dodecylhydrate of aluminum methyl-ammonium                                    sulfate.                                                            ______________________________________                                    

According to various investigations made by the inventors it has beenfound that GMO and its crystallographic isomorphs, that is, .[.(R_(x)R'_(11x))₂ O₃ Mo_(11e) W_(e) O₃ .]..Iadd.(R_(x) R'₁ _(-x))₂ O₃ . 3 Mo₁_(-e) W_(e) O₃ .Iaddend. (where, R and R' are at least one element ofthe rare earths, x is a number of 0-1.0, and e is a number of 0-0.2) arecrystals of the ferroelectric and ferroelastic phase belonging to pointgroup mm2, have a curie temperature approximately at 1600° C. showirregular ferroelectric properties at temperatures ranging from thecurie point to very low temperatures, including room temperature, areinsoluble in water, resistive to moisture as well as desiccation, andhave a high mechanical strength. Further, the curie point thereof can belowered down to around room temperature by forming isomorphous solidsolutions. A crystal of the GMO crystal structure employed in thepresent invention belongs to the orthorhombic system of crystalcrystallography.

The unit cell dimensions of GMO used in this invention have beendetermined by using an X-ray goniometer and by an X-ray diffractionmethod, as follows:

a=10.38±0.005 A.

b=10.426±0.005 A.

c=10.709±0.005 A.

As to Eu₂ (MoO₄)₃,Tb₂ (MoO₄)₃, Dy.sub. 2 (MoO₄)₃ and Sm₂ (MoO₄)₃ whichare isomorphous of GMO, it has been found from the measurement by anX-ray diffraction method that the unit cell dimension along the axis ais different from that along the axis b in all of these crystals asshown in table II.

                  TABLE II                                                        ______________________________________                                        Material a (A.)      b (A.)      c (A.)                                       ______________________________________                                        Eu.sub.2 (MoO.sub.4).sub.3                                                             10.377± 0.005                                                                          10.472±0.005                                                                           10.655±0.005                              Gd.sub.7 (MoO.sub.4).sub.3                                                             10.388±0.005                                                                           10.426±0.005                                                                           10.709±0.005                              Dy.sub.2 (MoO.sub.4).sub.3                                                             10.331±0.005                                                                           10.346±0.005                                                                           10.603±0.005                              Sm.sub.2 (MoO.sub.4).sub.3                                                             10.478±0.005                                                                           10.511±0.005                                                                           10.856±0.005                              ______________________________________                                    

Each single crystal of GMO, Sm₂ (MoO₄)₃, Eu₂ (MoO₄)₃, Tb₂ (MoO₄)₃ andDy₂ (MoO₄)₃ was cut in parallel with (100), (010), (001) planes whichare perpendicular to the axes a, b, c, respectively, and was subjectedto polling by being impressed with an electric field or a mechanicalstress to be made into a single domain structure. (This was verified byobserving 080) specimen through a polarizing microscope with planepolarized light directed in the direction of the axis c whilemanipulating a crossed polar.) The intensity distribution of lightreflected from the surfaces of the crystal was measured with an X-raythree axes goniometer. The planes the reflected light from which wasmeasured, were (400), (600), (800), (1000), and also (003), (004),(005). Further, after the measurement of the reflected light, the axes aand b of the crystal were interchanged by applying an inverse electricfield in the direction of the axis c or by applying a stress in thedirection of the axis c, and the crystal is made of a single domain.Then again, the intensity distribution of the light reflected fromplanes (040), (060), (080) and (0100) was determined under the followingmeasuring condition. That is, Cu-K rays from an X-ray source energizedwith a voltage of 30 kv. and a current of 10 ma. were directed to thecrystal through a divergence slit 10 mm. wide, a scattering slit 10 mm.wide and an entrance slit 0.1 mm. wide. The scanning speed of thegoniometer was one-fourth degree/min. and the radius of a geiger counterused was 185 mm. Further, when the crystal was heated above the curietemperature thereby to release it from the polled state and then cooled,it became of a multidomain structure and the difference between the celldimensions a and b became indistinct.

Some of the irregular ferroelectric crystals used in this invention aresingle crystals and solid solutions of chemical compounds of the GMOcrystal structure. Several of them have been shown in table I.

The structure of such a crystal is greatly affected by the size ofpositive ions contained therein. If the positive ions are too large ortoo small, a different structure will result. The Arrhenius ion radii ofions of rare earths are as follows: Sm.sup.⁺³ : 1.00 A. Eu.sup.⁺³ : 0.98A. Gd^(+:) 0.97 A. Tb: 0.93 A. and Dy: 0.92 A. Therefore, .[.(R_(x)R'_(11x))₂ O₃. 3Mo_(11e) WeO₃ .]. .Iadd.(R_(x) R'₁ _(-x))₂ O₃ . 3 Mo ₁_(-e) W_(e) O₃ .Iaddend. formed with these ion radii will have the sameGMO crystal structure.

The GMO crystal used in this invention belongs to the orthorhombicsystem and to the point group mm2 and has a spontaneous strain x_(S) asfollows: ##EQU1## A crystal having such unit dimension is remarkablyaffected by the polling. The GMO crystal used in this invention has thefollowing properties:

    ______________________________________                                        Color: Colorless and transparent                                              Density: 4,600 kg./m..sup.3                                                   Point group: Orthorhombic, mm2, ferroelectric phase                            at temperatures below the curie point; Tetragonal, 42 m.,                     paraelectric phase at temperatures above the curie point                     Phase transition temp.: 162°±3°C.                            Melting point: 1,170°C.                                                Cleavage plane: (110),(001)                                                   Specific dielectric constants in the direction of axes a, b and                c: ε.sub.c =10.5, ε.sub.a ≈ε.sub.b =9.5      (at 20°C.)                                                             Spontaneous polarization: 1.86×10.sup.-.sup.3 (C/M.sup.2)(axis c        direc-                                                                         tion)                                                                        Spontaneous strain: 1.5×(C/m.sup.2 10.sup.13)                           Elastic compliance: 25×10.sup.112 (m.sup.2 /Newton)                     Coercive field: 6×10.sup.5 (V/m.sup.2)                                  Coercive stress: 1.4×10.sup.5 (Newton/m.sup.2)                          Electrical resistivity: higher than 10.sup.10 Ω a                       Resistivity to water and chemicals: Good                                      Efflorescence and diliquescene: None                                          ______________________________________                                    

The following table III shows some of the isomorphs of GMO crystal usedin this invention. Reactive materials and the amounts thereof requiredfor forming the crystals also are shown in the table.

                                      TABLE III                                   __________________________________________________________________________                          Reactive material                                                             (mixture ratio)                                                               Molyb-                                                      Chemical formula of                                                                             date,                                                                              Rare                                               Number                                                                            single crystal    parts                                                                              earth Parts                                        __________________________________________________________________________    2   Sn.sub.2 (MoO.sub.4).sub.3                                                                      431.8                                                                              Sm.sub. 2 O.sub.3                                                                   348.7                                        3   Eu.sub.2 (MoO.sub.4).sub.3                                                                      431.8                                                                              Eu.sub.2 O.sub.3                                                                    352.0                                        4   Dy.sub.2 (MoO.sub.4).sub.3                                                                      431.8                                                                              Dy.sub.2 O.sub.3                                                                    373.0                                        5   Tb.sub.2 (MoO.sub.4).sub.3                                                                      833.6                                                                              Tb.sub.2 O.sub.3                                                                    748.8                                                                   Gd.sub.2 O.sub.3                                                                    180.9                                        6   (Gd.sub.0.5 Sm.sub.0.5).sub.2 (MoO.sub.4).sub.3                                                 431.8                                                                              Sm.sub.2 O.sub.3                                                                    174.3                                                                   Gd.sub.2 O.sub.3                                                                    180.9                                        7   (Gd.sub.0.5 Eu.sub.0.5).sub.2 (MoO.sub.4).sub.3                                                 431.8                                                                              Eu.sub.2 O.sub.3                                                                    176.0                                                                   Gd.sub.2 O.sub.3                                                                    180.9                                        8   (Gd.sub.0.8 Tb.sub.0.5).sub.2 (MO.sub.4).sub.3                                                  431.8                                                                              Tb.sub.4 O.sub.3                                                                    187.2                                                                   Gd.sub.2 O.sub.3                                                                    180.9                                        9   (Gd.sub.0.5 Dy.sub.0.5).sub.2 (MoO.sub.4).sub.3                                                 431.8                                                                              Dy.sub.2 O.sub.3                                                                    186.5                                                                   Gd.sub.2 O.sub.3                                                                    343.7                                        10  (Gd.sub.0.95 Yb.sub.0.68).sub.2 (MoO.sub.4 ).sub.3                                              431.8                                                                              Yb.sub.2 O.sub.3                                                                    19.7                                                                    Gd.sub.2 O.sub.3                                                                    343.7                                        11  (Gd.sub.0.95 Ho.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Ho.sub.2 O.sub.3                                                                    18.9                                                                    Gd.sub.2 O.sub.3                                                                    343.7                                        12  (Gd.sub. 0.93 Lu.sub.0.03).sub.2 (MoO.sub.4).sub.3                                              431.8                                                                              Lu.sub.2 O.sub.3                                                                    19.9                                                                    Gd.sub.2 O.sub.3                                                                    343.7                                        13  (Gd.sub.0.95 Tm.sub.0.03).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Tm.sub.2 O.sub.3                                                                    19.3                                                                    Gd.sub.2 O.sub.3                                                                    343.7                                        14  (Gd.sub.0.95 Sc.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Sc.sub.2 O.sub.3                                                                    6.9                                                                     Gd.sub.2 O.sub.3                                                                    343.9                                        15  (Gd.sub.0.95 La.sub. 0.05).sub.2 (MoO.sub.4).sub.3                                              431.8                                                                              La.sub.2 O.sub.3                                                                    16.3                                                                    Gd.sub.2 O.sub.3                                                                    343.9                                        16  (Gd.sub.0.95 Pr.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Pr.sub.5 O.sub.6                                                                    17.0                                                                    Gd.sub.2 O.sub.3                                                                    217.0                                        17  (Gd.sub.0.5 Y.sub.0.4).sub.2 (MoO.sub.4).sub.3                                                  431.8                                                                              Y.sub.2 O.sub.3                                                                     90.3                                                                    Gd.sub.2 O.sub.3                                                                    217                                          18  (Gd.sub.0.6 La.sub.0.4).sub. 2 (MoO.sub.4).sub.3                                                431.8                                                                              La.sub.2 O.sub.3                                                                    130.0                                                                   Gd.sub.2 O.sub.3                                                                    217                                          19  (Gd.sub.0.60 Tb.sub.0.20 Dy.sub.0.20).sub.2 (MoO.sub.4).sub.3                                   431.8                                                                              Dy.sub.2 O.sub.3                                                                    74.6                                                                    Tb.sub.4 O.sub.3                                                                    78.8                                         20  (Gd.sub.0.70 Eu.sub.0.20 Dy.sub.0.10).sub.2 (MoO.sub.4).sub.3                                        Gd.sub.2 O.sub.3                                                                    235.3                                                              431.8                                                                              Eu.sub.2 O.sub.3                                                                    70.4                                                                    Dy.sub.2 O.sub.3                                                                    37.3                                         21  (Gd.sub.0.60 Sm.sub. 0.20 Tb.sub.0.10).sub.2 (MoO.sub.4).sub.3                                       Gd.sub.2 O.sub.3                                                                    217.0                                                              431.8                                                                              Sm.sub.2 O.sub.3                                                                    69.7                                                                    Tb.sub.4 O.sub.7                                                                    39.4                                                                    Gd.sub.2 O.sub.3                                                                    253.3                                        22  (Gd.sub.0.70 Eu.sub.0.20 Tb.sub.0.10).sub.2 (MoO.sub.4).sub.3                                   431.8                                                                              Eu.sub.2 O.sub.3                                                                    70.4                                                                    Tb.sub.4 O.sub.7                                                                    39.4                                                                    Gd.sub.2 O.sub.3                                                                    253.3                                        23  (Gd.sub.0.7 Y.sub. 0.2 La.sub.0.4).sub.2 (MoO.sub.4).sub.3                                      431.8                                                                              La.sub.2 O.sub.3                                                                    32.6                                                                    Y.sub.2 O.sub.3                                                                     45.2                                                                    Gd.sub.2 O.sub.3                                                                    253.3                                        24  (Gd.sub.0.7 Eu.sub.0.20 Ho.sub.0.10).sub.2 (MoO.sub.4).sub.3                                    431.8                                                                              Eu.sub.2 O.sub.3                                                                    70.4                                                                    Ho.sub.2 O.sub.3                                                                    37.8                                                                    Gd.sub.2 O.sub.3                                                                    253.3                                                                   Sm.sub.2 O.sub.3                                                                    34.9                                         25  (Gd.sub.0.7 Sm.sub.0.4 Eu.sub.0.4 Y.sub.0.4).sub.2 (MoO.sub.4).sub.3                            Eu.sub.2 O.sub.3                                                                   35.3                                                                          Y.sub.2 O.sub.3                                                                     22.6                                                                    Gd.sub.2 O.sub.3                                                                    343.7                                        26  (Gd.sub.0.95 Nd.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Nd.sub.2 O.sub.3                                                                    16.8                                                                    Gd.sub.2 O.sub.3                                                                    217.0                                                                   Tb.sub.4 O.sub.7                                                                    78.8                                         27  (Gd.sub.0.5 Tb.sub.0.2 Y.sub.0.1 La.sub.0.4).sub.2 (MoO.sub.4).sub.3                            431.8                                                                              Y.sub.2 O.sub.3                                                                     22.6                                                                    La.sub.2 O.sub.3                                                                    32.6                                         28  Gd.sub.2 (Mo.sub.0.95 W.sub.0.1 O.sub.4).sub.3 (MoO.sub.4).sub.3                                     Wo.sub.3                                                                            70.0                                                                    Sm.sub.2 O.sub.3                                                                    174.1                                        29  (Sm.sub.0.5 Eu.sub. 0.5).sub.2 (MoO.sub.4 ).sub.3                                               431.8                                                                              Eu.sub.2 O.sub.3                                                                    176.0                                                                   Sm.sub.2 O.sub.3                                                                    174.1                                        30  (Sm.sub.0.5 Dy.sub.0.5).sub.2 (MoO.sub.4).sub.3                                                 431.8                                                                              Dy.sub.2 O.sub.3                                                                    186.5                                                                   Sm.sub.2 O.sub.3                                                                    174.1                                        31  (Sm.sub.0.5 Tb.sub.0.5).sub.2 (MoO.sub.4).sub.3                                                 431.8                                                                              Tb.sub.4 O.sub.7                                                                    187.5                                                                   Sm.sub.2 O.sub.3                                                                    331.3                                        32  (Sm.sub.0.95 Yb.sub.0.05).sub.2 (MoO.sub.4).sub.4                                               431.8                                                                              Yb.sub.2 O.sub.3                                                                    18.7                                                                    Sm.sub.2 O.sub.3                                                                    331.3                                        33  (Sm.sub.0.95 Ho.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Ho.sub.2 O.sub.3                                                                    18.9                                                                    Sm.sub.2 O.sub.3                                                                    331.3                                        34  (Sm.sub.0.95 Lu.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Lu.sub.2 O.sub.3                                                                    19.9                                                                    Sm.sub.2 O.sub.3                                                                    331.3                                        35  (Sm.sub.0.95 Tm.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Tm.sub.2 O.sub.3                                                                    19.3                                                                    Sm.sub.2 O.sub.3                                                                    331.3                                        36  (Sm.sub.0.95 Sc.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Sc.sub.2 O.sub.3                                                                    6.9                                                                     Sm.sub.2 O.sub.3                                                                    331.3                                        37  (Sm.sub.0.95 Y.sub.0.03).sub.2 (MoO.sub.4 ).sub.3                                               431.8                                                                              Y.sub.2 O.sub.3                                                                     11.3                                                                    Sm.sub.2 O.sub.3                                                                    313.4                                        38  (Sm.sub.0.90 Er.sub.0.4).sub.2 (MoO.sub.4).sub.3                                                431.8                                                                              Er.sub.2 O.sub.3                                                                    19.1                                                                    Sm.sub.2 O.sub.3                                                                    209.4                                        39  (Sm.sub.0.5 Eu.sub.0.3 Er.sub.0.4).sub.2 (MoO.sub.4).sub.3                                      431.8                                                                              Er.sub.2 O.sub.3                                                                    105.4                                                                   Er.sub.2 O.sub.3                                                                    19.1                                                                    Sm.sub.2 O.sub.3                                                                    244.0                                        40  (SM.sub.0.7 Tb.sub.0.2 Y.sub.0.4).sub.2 (MoO.sub.4).sub.3                                       431.8                                                                              Tb.sub.4 O.sub.7                                                                    78.8                                                                    Y.sub.2 O.sub.3                                                                     22.6                                                                    Sm.sub.2 O.sub.3                                                                    278.9                                        41  (Sm.sub.0.8 Er.sub.0.4 Y.sub.0.4).sub.2 (MoO.sub.4).sub.3                                       431.8                                                                              Y.sub.2 O.sub.3                                                                     22.6                                                                    Er.sub.2 O.sub.3                                                                    19.1                                                                    Sm.sub.2 O.sub.3                                                                    278.9                                                                   Dy.sub.2 O.sub.3                                                                    37.3                                         42  (Sm.sub.0.8 Dy.sub.0.1 Y.sub.0.05 Er.sub.0.05).sub.2 (MoO.sub.4).sub.3        5                 431.8                                                                              Y.sub.2 O.sub.3                                                                     11.3                                                                    Er.sub.2 O.sub.3                                                                    9.5                                                                     Wo.sub.3                                                                            70.0                                         43  (Sm.sub.0.5 Tb.sub. 0.5).sub.2 (Mo.sub.0.90 W.sub.0.1).sub.3                                    388.6                                                                              Sm.sub.2 O.sub.3                                                                    174.1                                                                   Tb.sub.4 O.sub.7                                                                    187.2                                                                   Dy.sub.2 O.sub.3                                                                    369.3                                        44  (Dy.sub.0.95 La.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              La.sub.2 O.sub.3                                                                    16.3                                                                    Dy.sub.2 O.sub.2                                                                    369.3                                        45  (Dy.sub.0.95 Pr.sub.0.05).sub.2 (MoO.sub.4).sub.3                                               431.8                                                                              Pr.sub.5 O.sub.11                                                                   17.0                                                                    Nd.sub.2 O.sub.3                                                                    16.8                                         46  (Dy.sub..95 Nd.sub.0.05).sub.2 (MoO.sub.4).sub.3                                                431.8                                                                              Dy.sub.2 O.sub.3                                                                    369.3                                                                   Dy.sub.2 O.sub.3                                                                    298.4                                        47  (Dy.sub.0.3 Nd.sub.0.10 Ho.sub.0.40).sub.2 (MoO.sub.4).sub.3                                    431.8                                                                              Ho.sub.2 O.sub.3                                                                    37.8                                                                    Nd.sub.2 O.sub.3                                                                    33.7                                                                    Eu.sub.2 O.sub.3                                                                    211.2                                        48  (Eu.sub.0.5 Tb.sub.0.04 Dy.sub.0.2).sub.2 (MoO.sub.4).sub.3                                     431.8                                                                              Dy.sub.2 O.sub.1                                                                    74.6                                                                    Tb.sub.4 O.sub.8                                                                    102.4                                                                   Gd.sub.2 O.sub.3                                                                    217.0                                                                   Sm.sub.2 O.sub.3                                                                    34.9                                         49  (Gd.sub.0.5 Eu.sub.9.7 Sm.sub.0.1 Tb.sub.0.4 Dy.sub.0.1).sub.1                                  431.8                                                                              Eu.sub.2 O.sub.3                                                                    70.4                                             (MoO.sub.4).sub.3      Dy.sub.2 O.sub.3                                                                    37.3                                                                    Tb.sub.4 O.sub.7                                                                    39.4                                         __________________________________________________________________________

The irregular ferroelectric crystals as listed above are positivelybiaxial and birefringent in the ferroelectric phase. FIG. 3 shows a partof the indicatric ellipsoid (or, in other words, light velocityellipsoid) of such a crystal. In FIG. 3, axes X, Y, Z indicateoptoelastic principal axes, and n.sub.α, n.sub.β, n.sub.γ indicaterefractive indices of light vibrating parallel to the axes, X, Y, Z,respectively.

In a GMO crystal, the optoelastic principal axes X, Y, Z coincide withthe crystallographic axes a, b, c, respectively. The crystal isuniaxially birefringent at temperatures above the curie point(approximately 160°C.), and its refractive indices with respect tosodium D line, λ=5,893 A, at 200°C. are as follows: ##EQU2## The crystalshows the irregular ferroelectric characteristics at temperatures belowthe curie point and becomes biaxially birefringent.

The optical axial angle 2 V (two times the angle V in FIG. 3) andrefractive indices n.sub.α, n.sub.β, n.sub.γ of the crystal against Na-Dline at room temperature are as follows: ##EQU3## The optical axialplane of this biaxial positive crystal is the crystallographic a plane(100), and this plane will rotate 90° around the axis c if the crystalis polarized reversely. Consequently, as is evident from FIG. 3, theretardation R_(a) of light transmitted through the GMO crystal in thedirection of the axis a is given by the formula, ##EQU4## where d.sub.αis the thickness of the crystal in the direction of axis a. If apolarization reversal occurs in such a crystal and the optoaxial planerotates 90° around the axis c, the axis a is replaced by the axis b andthe axis b by the axis a. Therefore, the above-mentioned retardationalso changes to the following value. ##EQU5## (The charge in thethickness of the crystal is due to the deformation of the unit cellequivalent to the 90° rotation of the axes a and b of the cell.) Thatis, the thickness as well as refractive index of the crystal changeswith the polarization reversal and accordingly the retardation alsochanges.

The retardation across a distance d of the light incident upon thecrystal in the direction at an angle θ to the axis c, for example, isd(nβ -n.sub.α '). In this case, if the crystal is reversely polarized,the above-mentioned retardation will become d(n.sub.β'-n.sub.γ) which isequivalent to the retardation of light propagating in the direction oc"which is on the plane ac and makes the angle θ with the axis c as shownin FIG. 3, since it can be deemed that the optoaxial plane (a-plane) ofthe crystal has been rotated 90° around the axis c.

If a crystal 3 as described above is positioned between two parallelpolarizing plates 1 and 2 as shown in FIG. 4 and white light 4 isdirected perpendicularly to the polarizer 1, the white light 4_(o)linearly polarized through the polarizer 1 is refracted by thebirefringence of the crystal 3 in various degrees depending on thecomponent wavelength thereof, becoming circularly polarized light at acertain frequency, linearly polarized light at another frequency andelliptically polarized light at the other frequencies. Of theelliptically polarized light, only the light having the same vibrationplane as that of the analzyer 2 passes through the analyzer 2 and givesan interference color. It should be noted that if the crystal isreversely polarized, thereby varying the retardation as describedpreviously, the above-mentioned interference color also varies accordingto the change in the retardation.

As stated above, irregular ferroelectric crystals such as GMO arebiaxially and positively birefringent. Consequently, if monochromaticparallel rays of light 4 are directed to an arrangement where in a Z-cut(cut perpendicularly to the c-axis) plate 3 of the crystal is disposed,as shown in FIG. 4, between a ploarizer 1 and an analyzer 2 thepolarization planes of which are perpendicular to each other, aninterference pattern as shown in FIG. 5 is formed on a screen. Theinterference pattern of FIG. 5 is loci of interference images formedaccording to whether the difference between the optical paths of therefracted rays of the monochromatic rays of light (wavelength: λ) passedthrough the crystal plate 3 is an even number times the half wavelength1/2λ or an odd number times the half wavelength 1/2λ. The phasedifference R between two extraordinary rays is ##EQU6## where d is thethickness of the crystal plate 3, and n_(o) and n_(e) are refractiveindices of the extraordinary rays, respectively. The spaces between theinterference fringes depend on the thickness d of the crystal plate, andbecome narrower as the thickness of the crystal plate is larger.

Since the refractive index varies with the wavelength, the positions ofthe interference fringes of FIG. 5 vary with the wavelength.

If the Z-cut crystal plate 3 in FIG. 4 is provided with transparentelectrodes 6 on both its Z-planes, i.e., c-planes, and if the crystalplate 3 is rotated around the c-axis so that the optoaxial plane thereofcoincides with the vibration plane of the polarizer, the screen becomesdark. A diaphragm or stop may be employed in this arrangement to improvethe parallelism of rays of light and the variation in brightness.

If the crystal plate 3 is rotated an angle θ from the dark state of thescreen around the c-axis, the relation between the quantity of thetransmitted light I and the rotation angle ##EQU7## where I_(o) is thequantity of the transmitted light when θ=π/4. Thus, when ##EQU8##

When the spontaneous polarization of the crystal 3 is reversed by theapplication of a negative voltage, the optical axis plane thereofrotates 90°. The variation in the quantity of the transmitted light atthis time is the same as that when the crystal is rotated 90° around thec-axis, and if the spontaneous strain is neglected, the variation in thebrightness of the transmitted light can easily be detected. α changesdepending on the angles between the plane perpendicular to the opticalaxis and the a- and b-axes.

Consequently, if the analyzer 2 is removed, the vibrating direction ofincident linearly polarized light can be rotated 90°. If the analyzer 2is employed, the quantity of the transmitted light can be varied betweenabundant and scanty states by a voltage at least equal to the coercivefield.

If the crystal 3 in FIG. 4 is one which is cut perpendicularly to theoptical axis, the birefringence does not take place in the direction ofthe optical axis. However, if the polarization state of the crystal isreversed, the birefringence takes place since the optoaxial planerotates 90°.

By providing transparent electrodes in the matrix form to both surfacesof an irregular ferroelectric crystal plate element such as a GMOcrystal element, and by applying a required voltage (at least equal tothe coercive field of the crystal) to each transparent electrode,required information can be stored at each position of each matrixelement in terms of "1" corresponding to the +P_(s) state of thespontaneous polarization as shown in FIG. 1b and "0" corresponding to-P_(s) state. If light is passed through such elements each storinginformation, the quantity of the light passed through the elementsdiffers from element to element depending on the polarized statethereof. Thus, the stored information can be read out nondestructivelywith light. Such readout is of a high S/N ratio, and a small sized largecapacity storage device can be made of an irregular ferroelectriccrystal.

Some embodiments of the invention will now be described.

EXAMPLE 1

As shown in FIG. 7, a plate 3 of GMO single crystal in Z-cut with athickness of 0.65 mm. provided with transparent electrodes 6 formed of,for example, SnO₂ or InO₂ on each c-plates thereof having an area 10 mm.× 10 mm. is disposed between a polarizer 1 and an analyzer 2. The b-axisof the crystal 3 forms an angle 25° with the vibration direction of thepolarizer 1 and the angle between the vibration directions of thepolarizer 1 and the analyzer 2 is 45°. Collimated monochromatic light 4of a wavelength λ=550 mμ is directed to the crystal plate 3 through thepolarizer 1. The light 4 changes into linearly polarized light 4_(o) bypassing through the polarizer 1. At this time, if the voltage (300volts) applied to the crystal plate 3 is adjusted by means of acontroller associated with a voltage source 7, the arrangement can beused as a light modulator or an optical shutter. The relation betweenthe quantity of the transmitted light and the applied voltage is asshown in FIG. 1d. Alternatively, if the analyzer 2 is eliminated fromthe arrangement of FIG. 7, the arrangement is used as a polarizationplane rotating element for rotating the polarization plane of linearlypolarized light by 90°.

EXAMPLE 2

If, a crystal plate 3, having an appropriate thickness is used,interference fringes as shown in FIG. 5 are observed. The crystal plate3 employed in this embodiment is arranged in such a manner that theoptical axis of the crystal 3 coincides with or is slight oblique to theoptical axis of the entire arrangement. Or alternatively, a crystal 3cut perpendicularly to the optical axis thereof and provided withtransparent electrodes on both cut surfaces is employed and light isdirected thereto perpendicularly or slightly obliquely to the cutsurfaces. Then, the quantity of transmitted light is null, or whenpolarization reversal is caused by applying an electric field thereto,the quantity of the transmitted light increases.

Analogously to ferroelectrics ferroelastics are conceivable. Materialshaving two or more states (orientations) of different strain in theabsence of any stress and capable of performing transition between thesestates by the application of strain are called ferroelastics herein.Ferroelastics generally have rectangular strain χ versus stress Xhysteresis loops in the absence of applied electric field similar to thehysteresis loops shown in FIGS. 1a to 1c.

In FIGS. 1a or 1b, curve AC corresponds to one oriented state, and curveDB corresponds to the other oriented state. The former is called "1 "state and the latter is called "0" state herein. One-half of thedifference between polarizations in both states or P_(s) or one-half ofthe difference between strains or χ_(x) is the absence of both electricfield and stress are called spontaneous polarization and spontaneousstrain, respectively. The electric field E_(c) and stress χ_(c)necessary for the transition from "0" state to "1" state or vice versaare called coercive field and coercive stress, respectively.

.[.195.]. Irregular ferroelectrics such as GMO are not onlyferroelectrics, but also ferroelastics. The kind and direction of anapplied stress for the transistion of ferroelastic state are as follows:If the z-axis is established parallel to the 4 symmetry axis in theordinary elastic phase (the phase above the curie temperature), and thex- and y-axes are established perpendicularly to two symmetry planes, aunit cell in the ferroelectric phase (the phase below the curietemperature) orientates as shown by A and B in FIG. 1a or 1b in "0"state and in "1" state. Therefore, in order to make transition from "0"state to "1" state, it may well be that a pressure is applied to thecrystal plane perpendicular to the x-axis and/or a tension is applied tothe crystal plane perpendicular to the y-axis. Or it may be good toapply shearing to the crystal along two pairs of crystal planes formingan angle of 45° with both x- and y-axes. In order to make transitionfrom "1" state to "0" state, it may be good to apply a pressure to thecrystal plane perpendicular to the y-axis, and/or to apply a tension tothe crystal plane perpendicular to the x-axis. Or it may be good toapply shearing opposite to the above-mentioned shearing to the crystalalong two pairs of crystal planes forming an angle of 45° with both x-and y-axes. .Iadd.

FIG. 12 illustrates a crystal 3 to which mechanical stress applyingmeans 20 is coupled to apply forces as shown by the arrows f, inconjunction with the above description. .Iaddend.

Even if the configuration of the crystal element is such that there isno crystal face perpendicular to or forming an angle of 45° with the x-or y-axis, it is possible to cause a transition of state by a stress.The kind and direction of an effective applied stress are determined asthe case may be.

Since GMO has a spontaneous polarization the direction of which varieswith the transition of state, the spontaneous polarization is apt toelectrostatically react to the transition of state due to stress.However, this reaction can be eliminated by applying a pair ofelectrodes to appropriate crystal faces and by short-circuiting them.

The spontaneous strain χ_(s) of GMO is defined by ##EQU9## where χ₁₁ andχ₂₂ are expansion coefficients of the crystal in the x- andy-directions, respectively.

Ferroelastics other than GMO are:

Potassium dihydrogen phosphate

Kh₂ po₄ (-150°c. or lower)

Dideuterate of ammonium arsenate

(ND₄)D₂ AsO₄ (27°C. or lower)

Rochelle salt

KnaC₄ H₄ O₆.4₂ O (between 24°C. and -180°C. inclusive)

Cadmium ammonium sulfate

(NH₄)₂ Cd₂ (SO₄)₃ (-178°C. or lower)

Dodecylhydrate of aluminum methyl-ammonium sulfate

(-96°C. or lower)

Generally, ferroelectrics vary in their refractive index by thetransition of state.

An embodiment of the invention based on the above-described property offerroelastics will now be described.

EXAMPLE 3

A storage unit 3 is disposed between a polarizer 1 and an analyzer 2 thepolarization planes of which are perpendicular to each other as shown inFIG. 8. The storage unit 3 is cut out from a GMO single crystal in sucha manner that its two main surfaces are perpendicular or slightlyoblique to its optical axis with a distance of 100 microns therebetween.The storage unit 3 is then provided on its main surfaces, after the mainsurfaces are polished, with groups of transparent electrodes 8, 8', 8",--; 9, 9', 9", --made of SnO₂ or InO₂ each having a width of 1 mm. Thegroups of electrodes 8, 8', 8",--; 9, 9,', 9", --are arranged so thatthey are in a row and column relation to each other as shown in FIG. 9.A voltage source 11 for supplying a negative voltage of one-half of thecoercive field E_(c) of the crystal is connected to the electrodes asshown in FIG. 10. Each group of the transparent electrodes consisted often electrodes in this embodiment, thus providing a 10×10 bits storagedevice having 10² storage elements.

Of course, a storage device having 10² storage elements is not a largecapacity storage device. Furthermore, the size 1 mm. × 1 mm. of theelement corresponding to one bit is rather large. If a large capacitystorage device of the order of 10⁶ bits, for example, is intended, thesize of the storage device will be large.

Since conventional phototransistors having a diameter 1 mm. wereemployed as detectors in this example, the number of storage elementswas limited to 10². If a large capacity storage device having, forexample, 10⁶ elements is desired, it may be well to form a number ofmicrominiature phototransistors in a crystal surface by integratedcircuit techniques.

The storage elements can store information by the application of adesired signal, for example a pulse of +120 volts with a duration of 10microseconds to the electrodes 8, 8', 8", --; 9, 9', 9", --. The readoutof the stored information is made by directing light through thepolarizer 1 to the storage device 3 and detecting the light passedthrough the element by the phototransistor 5 through the analyzer 2. Thelight passed through the element is strong when the element stores "1"and faint when it stores "0."

The above-described readout of stored information was in terms of ananalog quantity, i.e. brightness of light. However, the readout of thestored information can be made in terms of a digital quantity,wavelength of light.

In FIG. 4, if a GMO crystal 3 is arranged so that the z-axis thereof isin parallel with white light, it will be lightly colored. The GMOcrystal is biaxial at room temperature and the optical axes thereofintersect the x-axis symmetrically to each other. The optical axis angleof the GMO crystal is about 11 at room temperature and 0° at the curietemperature and becomes uniaxial.

By the arrangement of FIG. 4 at room temperature, interference fringesare observed around the two optoaxial points a and b shown in FIG. 5,and the surroundings of the interference fringes are colored. Theinterference fringes are considered to be loci of the retardation. Sincethe retardation R has the relation R=d(n_(e) ˜n_(o)) with the thicknessd of the crystal and the refractive indices n_(o) and n_(e) of the twoextraordinary rays, the difference Δn=n_(e) ˜n_(o) between therefractive indices is zero in the direction of the optical axis, and thedifference Δn becomes larger as the departure from the optical axisbecomes larger.

The interference color is determined by the retardation R. Bright colorsresult on the interval of the retardation R of 400 mμ and 800 mμ. Whenthe retardation R is in the vicinity of 800 mμ, the color is red, andwhen the retardation R is near to 400 mμ, the interference color isblue. Since the difference Δn varies with the solid angle around theoptical axis at a given thickness of a crystal, the color of the lighthaving passed through the crystal varies accordingly. Consequently, ifthe optical axis of the crystal is appropriately inclined relative torays of light in accordance with the thickness of the crystal, a desiredcolor of light can be obtained. If the crystal is fixed and theoptoaxial plane is rotated by the polarization reversal, the color oflight generally changes. It is easier to discern the two colors when thewavelengths thereof are different as far as possible.

The angle of the optical axis of the crystal relative to incident lightcan effectively be selected by observing the interference color which isthe locus of the retardation R shown in FIG. 5. For example, if ac-crystal plate 0.2 mm. thick is set at 11° in the direction of the axisb, and 7° in the direction of the axis a in the single domain state, thecolor is red in the +P_(s) state and blue in the -P_(s) state.

Therefore, if the storage device 3 in FIG. 8 is replaced by a storagedevice made of such a crystal, the contents of the store can directly beidentified. Further, if photodiodes having different sensitivity to twowavelengths indicating the contents of store are employed, or ifphotodiodes having sensitivity only to either one of the wavelengths areemployed, the contents of the store can be read out with an electricalsignal having a good signal to noise ratio.

The signal to noise ratio of the readout signal can greatly be increasedby inserting a quarter wavelength plate 10 for the central wavelength ofwhite light between the analyzer 2 and the phototransistors 5 in FIG.10.

As has been described above, the storage device according to thisinvention is made of an irregular ferroelectric or ferroelastic materialsuch as GMO, and the information stored in the storage elements of thestorage device is read out by passing polarized light through thestorage elements.

When a ferroelectric material is employed as the storage device, thereare the advantages that (1) the power consumption of the storage elementis small, and (2) a small sized large capacity storage device can befabricated since the storage density can be made large.

However, since a storage device employing ferroelectric material storessignals as polarized states of its storage matrix elements correspondingto respective signals by being supplied with predetermined signals, theinformation stored in the storage elements is read out by being suppliedwith definite reverse voltage pulses. When a pulse as shown in FIG. 11ais fed to a storage element for such reading out, only a low current asshown in FIG. 11b flows through the storage element if the polarity ofthe pulse is the same as the polarized state of the element. However, ifthe pulse is of opposite polarity with a sufficiently large amplitude,the polarized state of the element is reversed, accompanied by arelatively high current as shown in FIG. 11c flowing through the storageelement to read out the information (i.e., polarized state) stored inthe element.

The ferroelectric materials conventionally employed for such storagedevice were barium titanate and glycine sulfate, for example. In theseferroelectrics, there exists no coercive field corresponding to thethreshold field E_(c) for reversing the polarized state in the P-Ehysteresis loop as shown in FIG. 1a. This is because, since the coercivefield generally has dependency on voltage, frequency, and time, even alow voltage pulse can cause the crystal to reverse its polarization whenit is applied to the crystal for a long time. That is, the coercivefield is substantially zero against a quasi-static change in electricfield, according to which the memory state of the crystal is apt to beunstable.

Further, since it is necessary to apply a pulse of reverse voltage to astorage element in order to read out the information stored therein, thestored information is destroyed due to the polarization reversal.Consequently, stored information cannot repeatedly be read out.

Still further, in such a reading method, all the elements of the i-throw and the j-th column are impressed with one-half the negative voltagenecessary for reading out an element (the threshold voltage) in order toread the element at (i,j), for example. Although this voltage is smallerthan the threshold value necessary for polarization reversal, i.e., thecoercive field, the polarization reversal occurs gradually to cause anoise current since the coercive field of the conventional ferroelectricmaterial has not a definite threshold value. Even when the polarizationreversal does not occur but merely a charging current flows, the currentbecomes a cause of noise, and hence the S/N ratio becomes low and alarge capacity storage device is difficult to fabricate.

However, if an optical shutter element utilizing the change in polarizedstate of an irregular ferroelectric or ferroelastic material such as GMOis employed as a storage element as in the present invention,nondestructive readout can be effected and the S/N ratio of readout ismade large, thus making it possible to fabricate a large capacitystorage device.

We claim: .[.1. A device for modulating a beam of light comprising apair of light polarizer plates disposed substantially in parallel witheach other and having their surfaces substantially perpendicular to thedirection of incident light thereon an irreguar ferroelectric elementhaving a pair of Z-cut planes, said irregular ferroelectric elementbeing arranged between said pair of light polarizer plates in such amanner that said Z-cut planes of said element are substantially parallelto said light polarizer plates, a pair of transparent electrodes eachprovided on each of said pair of Z-cut planes, and means for applying anelectric field not lower than the coercive field of said element acrosssaid element through said pair of transparent electrodes..]. .[.2. Adevice according to claim 1, further comprising a quarter wavelengthplate for the central wavelength of white light..]. .[.3. A deviceaccording to claim 1, wherein said irregular ferroelectric material is asingle crystal having molybdate gadolinium oxide structure representedby (R_(x) R'_(11x))₂ O₃.3Mo_(1-e) W_(c) O₃, where R and R' are at leastone element of the rare earths, x is a number of from 0 to 1.0, and e isa number of from 0 to 0.2..]. .[.4. A device according to claim 1,wherein each of said pairs of transparent electrodes comprises aplurality of parallel strips at equal intervals, said strips comprisingoppositely biased electrodes crossing over substantially perpendicularlyto each other..]. .[.5. A device according to claim 4, wherein saidelectrodes are made of one of SnO₃ and InO₂..]..Iadd.
 6. A device formodulating a beam of light comprising: a pair of light polarizer platesdisposed substantially in parallel with each other; an irregularferroelectric element having a pair of Z-cut planes; a pair oftransparent electrodes each provided on each of said pair of Z-cutplanes; and means for applying an electric field not lower than thecoercive field of said element across said element through said pair oftransparent electrodes, said irregular ferroelectric element being madeof molybdate gadolinium oxide crystal structure given by the formula

    R.sub.2 O.sub.3 . 3 Mo.sub.1.sub.-e W.sub.e O.sub.3

where e is a number of from 0 to 0.2, and R is an element selected fromthe group consisting of Sm, Tb, Dy and Eu. .Iaddend. .Iadd.
 7. A devicefor modulating a beam of light comprising: a pair of light polarizerplates disposed substantially in parallel with each other; an irregularferroelectric element having a pair of Z-cut planes; a pair oftransparent electrodes each provided on each of said pair of Z-cutplanes; and means for applying an electric field not lower than thecoercive field of said element across said element through said pair oftransparent electrodes, said irregular ferroelectric element being madeof molybdate gadolinium oxide crystal structure given by the formula

    (R.sub.x R'.sub.1.sub.-x).sub.2 O.sub.3.3Mo.sub.1.sub.-e W.sub.e O.sub.3

where R is constituted by at least one element selected from the groupconsisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Eu, Tb, Tm, Yb, and Lu, R'is constituted by at least one element selected from the groupconsisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Gd, Eu, Tb, Tm, Yb, and Lu,x is a number of from 0 to 0.1 and e is a number of from 0 to 0.2..Iaddend. .Iadd.
 8. A birefringent device comprising: a crystal elementmade of molybdate gadolinium oxide crystal structure given by theformula

    R.sub.2 O.sub.3.3 Mo.sub.1.sub.-e W.sub.e O.sub.3,

where e is a number of from 0 to 0.2, and R is an element selected fromthe group of Sm, Tb, Dy and Eu; and means, connected to said crystal,for placing said crystal in one of the reversibly birefringent statesthereof. .Iaddend..Iadd.
 9. A device according to claim 8, wherein saidmeans for placing said crystal in one of the reversibly birefringentstates thereof comprises means for imparting a mechanical stress to thecrystal at least equal to the coercive stress thereof. .Iaddend. .Iadd.10. A device according to claim 8, wherein said means for placing saidcrystal in one of the reversible birefringent states thereof comprisesmeans for applying an electric field across said crystal at least equalto the coercive field of said crystal. .Iaddend..Iadd.
 11. A deviceaccording to claim 10, wherein said means for applying an electric fieldcomprises a voltage source and a pair of electrodes connected to saidcrystal for receiving the voltage from said voltage source and applyingsaid electric field across said crystal. .Iaddend..Iadd.
 12. A deviceaccording to claim 10, wherein said means for placing said crystal inone of the reversibly birefringent states thereof comprises means forapplying a voltage pulse to said crystal. .Iaddend..Iadd.
 13. Abirefringent device comprising: a crystal element made of molybdategadolinium oxide crystal structure given by the formula

    (R.sub.x R'.sub.1.sub.-x).sub.2 O.sub.3.3Mo.sub.1.sub.-3 W.sub.e O.sub.3

where R is constituted by at least one element selected from the groupconsisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Eu, Tb, Tm, Yb, and Lu, R'is constituted by at least one element selected from the groupconsisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Gd, Eu, Tb, Tm, Yb, and Lu,x is a number of from 0 to 0.1 and e is a number of from 0 to 0.2; andmeans connected to said crystal, for placing said crystal in one of thereversibly birefringent states thereof. .Iaddend..Iadd.
 14. A deviceaccording to claim 13, wherein said means for placing said crystal inone of the reversibly birefringent states thereof comprises means forimparting a mechanical stress to the crystal at least equal to thecoercive stress thereof. .Iaddend..Iadd.
 15. A device according to claim13, wherein said means for placing said crystal in one of the reversiblebirefringent states thereof comprises means for applying an electricfield across said crystal at least equal to the coercive electric fieldof said crystal. .Iaddend..Iadd.
 16. A device according to claim 15,wherein said means for applying an electric field comprises a voltagesource and a pair of electrodes connected to said crystal for receivingthe voltage from said voltage source and applying said electric fieldacross said crystal. .Iaddend..Iadd.
 17. A device according to claim 15,wherein said means for placing said crystal in one of the reversiblybirefringent states thereof comprises means for applying a voltage pulseto said crystal. .Iaddend.